NOV 2 2 1994 National Capital Astronomers, Inc. 3011320-3621 Volume 53, Number 4 December, 1994 ISSN 0898-7548 Virginia Trimble to Speak on "The Universe You Don't See: Existence and Nature of Dark Matter" by Harold Williams The next meeting of the National Capi- Matter Problem." The 5th Annual As- The mathematics needed to understand tal Astronomers will be held on Satur- trophysics Conference in Maryland Oc- this in detail are only what is required in day, December 3rd, at 7:30P.M., in the tober to through 12 at College Park was the first part of any physics class. How Bunim room on the ninth floor of the devoted to this topic this year. There is many scientific fields can you be put on Clinical Center (Building to) at the much diverse evidence leading to the the frontier when you take the first intro- National Institutes of Health (NIH). conclusion that most of the universe is ductory course? I think very few. Virginia Trimble of the University of dark. The most convincing evidence is Maryland will speak on "The Universe the flat rotation curves of disk galaxies. The mathematics follows. For those You Don't See: Existence and Nature of The argument goes as follows-stars in who this does not make physical and Dark Matter." The speaker sent the disk galaxies move in predominately algebraic sense you may skip to the end following abstract: "Many different circular orbits. Newton's Laws and of the article where the speaker is de- lines of observational evidence indicate Newtonian Universal Gravity tell us scribed. strongly that 90% or more of the gravi- that the mass enclosed in a spherical or tational potential of the universe comes cylindrical symmetric is determined by Newton's Universal Law of Gravity: from matter which does not emit its fair the velocity ofthe orbiting bodies. The share of light or other electromagnetic mass, as measured in light and the veloc- F=GMm radiation. This is the infamous dark ity of stars, do not agree. The mass in R2 matter, and its existence has been firmly stars in the center of most galaxies de- where 1':is the force, G is the Newtonian established for nearly 20 years. But we creases as a power law to the minus 3.4 gravitational constant,Mis one mass, m know remarkably little about what it is. power. The mass, in stars in the disk of is the other mass (the one whose accel- Candidates range from neutrinos with most galaxies decreases exponentially. eration we will consider), and R is the rest masses of a few eV through other, The mass as measured by its doppler distance between M and m. even stranger particles, to substellar shifted light of the stars whirling around objects and 106 solar mass black holes. the galaxy, in fact, not only does not Since gravity is a central force, it acts A range of laboratory experiments and decrease like the mass in luminous stars, only on the mass internal to radius R, if astronomical searches, currently in but it increases linearly with the distance the mass distribution is symmetric in R. progress, could identify the most likely away from the galaxy. The dark matter Since disk galaxies are symmetric inR, candidate(s) in the near future, but could phenomenon is indeed this glaring error we can write the force on a star of mass also leave the issue unresolved indefi- between these two different techniques mas: nitely." for measuring mass, by it luminous ef- F=GM(R)m fect and by its gravitational potential, as R2 The largest unsolved problem in all of evidenced by the motions of the stars astronomy and physics is the "Dark that we see doppler shifted. See Trimble, Page 3 :I: VeCem6e~Calendar The Public is Welcome! Friday, December 2, 9, and 30,1994,8:30 PM-Open Washington-Baltimore region. Information: USNO nights with NCA's Celestron 14-inch (0.36 meter) Public Affairs Office, 202/653-1541. telescope at Ridgeview Observatory, 6007 Ridgeview Drive (off Franconia Road between Telegraph Road Tuesdays, December 6,13,20, and 27,1994,7:30 andRoseHill Drive). Information: Bob Bolster at 703/ PM-Telescope making classes at Chevy Chase Com- munity Center, Connecticut Avenue and McKinley 960-9126. Street, NW. Information: Jerry Schnall, 202/362- Fridays, December 2, 9,16,23, and 30,1994,7:30 8872. PM-Telescope making classes at American Univer- sity, McKinley Hall Basement. Information: Jerry Wednesday, December 7, 1994-December "Sky Watch" column by Blaine P. Friedlander, Jr. appears Schnall, 202/362-8872. in The Washington Post "Style" section. It lists many Saturday, December 3, 1994, 7:30 PM-The Decem- other events for the month. berNCA meeting will feature Virginia Trimble speak- Saturday, December 17, 19947:30 PM-"Exploring ing about "The Universe You Don't See: Existence the Sky" telescope viewing at the open field in Rock and Nature of Dark Matter." Creek Park nearest to the Nature Center. NCA mem- Saturday, December 3,1994,5:30 PM-Dinner with bers please bring telescopes. For more information, the speaker at the La Posada Restaurant, 8117 call John Lohman, 703/820-4194 Woodmont Ave., Bethesda, MD., before the monthly Saturday, December 24, 1994, Night-December's meeting. Reservations are for 5:30 p.m., sharp. second best night for dark-sky observing and "absorb- Saturday December 3, 1994, Night (After The ing" ("Moon-dark" until around Midnight). See De- Meeting)-December's best Saturday night for dark- cember 3rd listing. sky observing, and "absorbing" (Moon sets by 8:15 Saturday, December 31, 1994, Night-December's p.m., providing nine hours of "Deep Night," weather third best night for dark-sky observing and "absorb- permitting, until dawn, Sunday morning). Several ing" ("Moon-dark" until around Midnight). See De- relatively light-pollution-free sites are available for cember 3rd listing. NCA members's use. Information: Daniel Costanzo, 703/841-4765. Friday, January 6, 1995, 7:00-8:30-Sandy Spring Friends School Presents, Star Party: Astronomy Ac- Mondays, December 5, 12, 19, and 26, 1994, 8:30 tivities. Free to the public. See the ad on page 5. PM-Public nights atU.S. Naval Observatory (USNO), in Northwest Washington, DC (off Massachusetts Saturday, January 7, 1995,7:30 PM-The January Avenue). Includes orientation program on USNO's NCA meeting will feature Dimitris Christodolou mission, viewing of operating atomic clocks, and speaking about "The Formation of Binary Stars glimpses through the finest optical telescopes in the through Fission of a Single Mass: Past and Present." Mark Your Calendar! - Next Six NCA Meetings Below are dates for the next six NCA meetings coming up after this December's meeting unless otherwise indicated. January 7 April 1 February 4 May 6 March 4 June 3 Page 2 TRIMBLE, from page I whereM(R) is the mass enclosed within remembering that an isothermal fellow of many learned societies and a radius R. sphere-uniform temperature sphere serves on advisory panels and commit- where the temperature causes pressure tees too numerous to mention. Her ser- Newton's second law of motion: which is balanced by the inward force of vice as an article reviewer, writer of gravity has a mass distribution propor- reviews, editor, and speaker are legion. F=ma. tional to r". What the creator is trying to She is currently the associate editor of The geometry of acceleration in a circle: tell us here has not been fully fathomed theAstrophysicalJoumal. Her publica- by anyone yet. tions are also legion. Her resume is so a(R) = x(R)2 well organized that I have never seen a R Our speaker, Virginia Trimble, is both better one. I am going to modify my Combining the equations: bicoastal and biacademic and is profes- resume along the lines of hers as soon as sor at the University of Maryland at I get time. Once you have heard her m L= GM(R)Y1l! College Park and the University of Cali- speak, you wiII go out of your way to R R2 fornia at Irvine. She is a member and hear her speak again. Solving for the velocity of a star m at radius R yields: x(R) =VG~(R) Keplerian motion results when all of the I '-' 200 mass is concentrated in the center so 1 x(R)=~ '3 ~ 100 This would make the velocity fall off proportional to ViTR. The velocity of the planets around the o 20,000 40,000 60,000 sun and the velocity of the moons Distance from center of galaxy (ly) around a planet follow Keplerian rota- tion to fairly -0gh order falling off pro- The rotation curves of stars within our own galaxy. (Kaufmann: UNIVERSE, Third portional to / R . Edition © 1991, W H. Freeman and Company) BUT Disk Galaxy rotation is FLAT. To first order it does not vary with radial distance except in the very center of the 350 galaxy. There is a huge difference be- tween decreasing proportional to ~ 300 NGC4378 and remaining constant or nearly flat. ---- 250 One can go one step further and invert ~ NGC 1620 "0 200 , NGC7664 the equation, solving for the mass within 0 a radius R and one finds 8. '" \ , ] 150 -, R :e -,, MIRJ =J;<r)4 wdr = constant R. 0 100 "<, '-...-...If ...•. - 50 So the mass density p of the dark matter --- is p(r) = Konstant . 0 5 10 15 20 25 r Distance from center of galaxy (kpc) At least the mass density does decrease, but a density of r2 falls off much slower The rotation curves offour spiral galaxies.